Challenges to the development of life

The development of complex life before this time may have been hindered by the atmospheric changes that the biota produced. The prior abundance of carbon dioxide in the atmosphere had provided an insulating, or greenhouse, effect. As organisms removed this gas from the atmosphere, the greenhouse effect was lessened and the Earth’s climate changed. This occurrence is believed to have resulted in severe ice ages that gripped the planet.

The causes of the ice ages are still hotly debated. One hypothesis proposed in 1990 by the geologist John James Veevers links their occurrence to continental drift. According to this model, continental drift is cyclic: in the past 1.2 billion years the continents have fluctuated between a phase in which all the Earth’s landmasses are separate and a “supercontinent” phase, in which these distinct landmasses formed one continent. During the supercontinent phase, little spreading of the seafloor, with its concomitant release of carbon dioxide from the Earth’s mantle, would have occurred. Thus, less carbon dioxide would be present in the atmosphere and the greenhouse effect would be lessened, creating a cooler environment. Major ice ages are believed to coincide with each of the supercontinent phases. (However, the ice ages of the past two million years, which were short-lived and oscillating, are not thought to be part of this larger cycle.)

The distribution of life-forms dependent on a nearby shoreline or a terrestrial habitat has been affected by the relative positions of the continents. The cyclic breakup of supercontinents has provided many opportunities for evolution to continue in isolation. Today Australia is the most isolated of the continents, and its unique flora and fauna are well known. In the past other landmasses have been equally if not more isolated. A part of what is now Central Asia, known as Kazakhstania, was an isolated landmass between the latter half of the Cambrian (about 513 to 488 million years ago) and the first half of the Devonian (about 416 to 385 million years ago). On these and other landmasses unique floras and faunas evolved (seebiogeographic region).

The diversification of life

Until the 1980s the fossil record of early multicellular organisms was interpreted to be one of simple and rapid diversification. The paleontologist Adolf Seilacher and others have argued that this is incorrect and that the earliest faunas of multicellular organisms include few or no species that are directly ancestral to later faunas. As evidence they point to the early fauna from the Ediacaran period—animals living at the end of the Precambrian era, between 700 and 542 million years ago, that were named after the Ediacara Hills in South Australia. Few of the Ediacaran fauna are believed to be related to the later fauna of the Burgess Shale of western Canada (from the middle of the Cambrian [about 520 to 500 million years ago]). In this view the fossil record is believed to have resulted from at least two more or less independent evolutionary radiations of multicellular organisms followed by severe extinction. Thus extensive extinctions would have played an important role in the evolution of life even at this distant period. Other authorities disagree with this model and maintain that the Ediacaran animals have relatives from the Phanerozoic Era (encompassing the Paleozoic, Mesozoic, and Cenozoic [542 million years ago to the present]), such as sea pens and polychaete worms.

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